1. Field of the Invention
The invention relates to a piezoelectric transducer in the form of a spheroidal cap, for the location and destruction of hard concretions within an animal body, more particularly the human body. Thus, it should be understood that the term "animal" is used generically herein to embrace humans and what are commonly referred to as animals.
2. Description of the Prior Art
With comminutions of brittle solids formed within the body, e.g. such as kidney, bladder or gall stones, it is impossible without having an internal operation to destroy the same except by means of focussed ultrasonic oscillation. However, the application of focussed ultrasonic waves to the body should be undertaken with care to ensure that injurious energy densities fall directly on the object which is to be destroyed and do not harm or destroy normal healthy tissues. To achieve this object, it is known to use for example spark gaps under water as sound sources, and to concentrate the ultrasonic emission on the locus of the concretion by means by an elliptically shaped reflector. This method has the disadvantage that the shock waves generated by spark gaps are reproducible only with difficulty and, consequently, may be metered also with difficulty, and that concentration on targets of minimum size is impossible in view of the size of the bubble formed during spark discharge. Furthermore, the bubbles produced have to be eliminated between two consecutive shock waves, and the spark gaps utilised have a very short service life only (e.g. 100 discharges).
A second known possibility consists in making use of ultrasonic transducers as sound sources, which either have the form of spheroidal caps or are focussed by application of lens systems. However, the greatest difficulty during application of ultrasonic transmitters consists in securing the high energy densities required. According to experience, pressure amplitudes of the order of magnitude of 2000 bar are needed for destruction of concretions. Lens systems are hardly applicable for this reason, because reflection and absorption in the lens material cause excessive losses. Ultrasonic transducers in the form of spheroidal caps are satisfactorily appropriate for the continuous emission of ultrasonic oscillation, but the application of continuous ultrasonic oscillation to a concretion formed within the body is impossible because burning of normal healthy body tissue in the vicinity of the concretion would be unavoidable at the high energy density required. In principle, shock waves may also be generated with ultrasonic transducers in the form of spheroidal caps, but this presupposes an extremely high load-bearing capacity of the transducer elements because the resonance increase of the oscillation amplitude occurring during continual energisation cannot be exploited whilst doing so. Ultrasonic transducers in the form of spheroidal caps are commonly produced as piezoceramic appliances, e.g. based on barium titanate, by being pressed into shape, sintered and then polarised radially. Since the variation in the thickness of the material caused by the action of the electrical charge applied is always combined with a transverse contraction at the same time, such spheroidal ceramic caps are destroyed very rapidly during pulse excitation at high voltages. Special measures are needed for this reason, to secure the high load-bearing capacity required.
On the other hand, piezoelectric transducers have the advantage that the pulses which they generate may be reproduced and metered perfectly and that their service life, subject to appropriate construction, is considerably greater than that of spark gaps. Another advantage of piezoelectric transmitters is that it is possible to utilise one and the same transmitter to generate the shock waves as well as to locate the concretion. Since different tissue structures have to be transirradiated between the surface of the body and the concretion, there is always the risk that the focus may be displaced by sound refraction, so that perfect alignment on the locus of the concretion, e.g. determined by X-rays, is possible. However, adjustment defects of this kind cannot arise, if ultrasonic pulses radiated at low power by the shock wave transducer itself are utilised for location.
Experience shows that it is inappropriate to expose the whole concretion which is to be destroyed to the shock wave at the same time, and that it is more advantageous to concentrate the power successively in chronological sequence on separate sections of the concretion. As a matter of fact, comparatively large fragments are formed in the first case, whereof the removal by natural means is frequently still impossible, whereas, in the second case, the concretion disintegrates into minute and almost dust-like fragments which may be removable by natural means. Accordingly, the main object of the invention consists in concentrating the sonic energy emitted by a piezoelectric transducer on a minimum cross-section and in limiting the required total output.